Typically gold ore is composed of up to a few ounces of gold per ton of host medium. Gold has a density of about 17 gm/cm.sup.3, and silica, a typical host medium, has a density of about 3 gm/cm.sup.3. Traditionally gold ore has been crushed into fine particles in a mill, then a cyanide leaching process is used to chemically remove the gold contained in the crushed ore. Once the gold is removed the crushed ore or tailings (as they are now called) are disposed of. Invariably this method is inefficient in that not all the gold is removed. The primary reason for this inefficiency lies in the fact that not all the gold is exposed to the leaching action due to inadequate crushing. This inadequate crushing is due to interference caused by the smaller particles protecting the larger particles which require further crushing.
Forms of an elutriation technique have been in use in laboratories for decades where particles are sorted according to diameter, as in a "Cyclo-sizer" (Trademark of Warman International).
The other major form of density separation previously used employed a stationary high density slurry into which material to be separated was simply dropped then vibrated. This batch method of density separation has proven to be too slow and expensive to be used on a large scale.
Two Canadian patents of interest relating to the use of fluids to separate mixed materials are Canadian Patent No. 102,673 of Trottier issued Dec. 18, 1906 and Canadian Patent No. 373,878 of Remick issued May 17, 1938. The Trottier reference describes and illustrates a single column apparatus for separating at different levels in the column material of constant size but of different density. The column contains a plurality of vertically spaced sorting tables and collection vanes which act to sort materials according to density and permit higher density materials to fall while lighter density materials are drawn off at appropriate heights within the column down-shoots. The Remick patent describes an apparatus and process for separating low density materials such as coal from a higher density host material such as slate. The mixed materials are introduced to a tank provided with an upwardly flowing current of water which carries particles of lesser specific gravity, such as coal, upwardly and out of the upper end of the tank onto an overflow wier, where they are collected on a screen. Again the Remick apparatus is inefficient over a large range of particle sizes.
The object of the invention is to apply a unique, controlled, combined elutriation screening technique to density separations, particularly applicable to gold and diamond tailings in order to economically recover previously unavailable valuable materials.